Centre for Extragalactic Astronomy

Cluster starbursts

A number of physical processes have been proposed to explain the strong correlations seen in the local Universe between the properties of galaxies and their environment.  In particular, the strong concentration of   passive, metal-rich spheroidal galaxies (ellipticals and S0s) in high-density regions (such as the cores of clusters of galaxies).  This association of these dispersion-dominated systems with high-density regions today suggests that dynamical interactions may in part responsible be responsible for these trends, but whether these are the sole explanation and exactly when they act are still unknown.  

Attempts to infer  the star-formation histories of local elliptical galaxies from analysis of the spectra of their stars suggests that they formed most of their stars 8-11 billion years ago, at redshifts above z=1.  Moreover, the fact that these galaxies are metal-rich also suggests that this activity may well have been obscured by dust. These studies also indicate that more massive ellipticals having older stellar populations. Thus  the available evidence  suggests that young spheroidal galaxies in regions which collapse to  become  clusters, must have formed their stars faster (and so at a higher star-formation rate, SFR) than those in less-dense regions.  Hence, while clusters today are less active than the surrounding field, their progenitor structures at earlier times were actually more active than lower-density regions. 

These suggestions are supported by the discovery in surveys of clusters at z>0.5 in the mid- and far-infrared and submillimeter, of a population of dusty ultra/luminous infrared galaxies (U/LIRGs). Such strongly starbursting galaxies are almost absent from such environments in the local Universe, but they demonstrate accelerated evolution which means that the cores of z<1 clusters host significant numbers of  dusty U/LIRGs.  

Our work focuses on bringing together dynamical and multiwavelength tools to investigate the properties of strongly star-forming galaxies in high density environments from z~0.5 to z~1.5 and beyond. We use a combination of dust-insensitive selection and three-dimensional imaging spectroscopy  to first select samples of starbursts in these environments and to then disentangle and understand their dynamics, distribution of star formation and gas masses. The Figure shows the velocity fields from 3-D integral field observations of starburst galaxies in a z~0.5 cluster, demonstrating that most have coherent disk-like kinematics, suggesting their enhanced activity is not triggered by major mergers.  By comparing these to predictions of numerical simulations of galaxy formation we will distinguish between the physical mechanisms responsible for the rapid evolution in this population.

Staff involved with this project at Durham include Richard Bower and Ian Smail.